Although not stated on-screen, the Klingon prison planet was meant to be Rura Penthe (which is actually an asteroid). A deleted scene from the film shows that Nero and his crew from the Narada were captured by the Klingons and imprisoned on Rura Penthe for twenty-five years after their ship was crippled by the USS Kelvin in 2233. The attack on the prison planet referenced in the film was Nero and his crew escaping from Rura Penthe and reclaiming their vessel.[1][2]